BEIRUT, July 11 (Reuters) - Syria's ambassador to Iraqdefected on Wednesday in protest over President Basharal-Assad's crackdown on a 16-month uprising as the U.N. SecurityCouncil remained deadlocked over the next steps in the crisis.

"I declare that I have joined, from this moment, the ranksof the revolution of the Syrian people," Nawah al-Fares said ina video statement posted on Facebook. He did not elaborate orsay from where he had posted the statement.

Fares, who has close ties to Syrian security, was the firstsenior diplomat to quit the embattled government.

There has been no comment from Damascus or Baghdad and theWhite House said it was unable to confirm the defection, hailedby Assad's opponents as a sign of crumbling support.

But Assad's chief backer at the U.N. Security Council,Russia, remained firmly in the Syrian leader's camp as the15-member group made little headway after international mediatorKofi Annan asked it to agree on "clear consequences" if thegovernment or opposition fail to comply with his faltering planfor a political solution to the crisis.

Fares, a veteran of Assad's rule who held senior positionsunder the late president Hafez al-Assad, is from Deir al-Zor,the eastern city on the road to Iraq which has been the scene ofa ferocious military onslaught by Assad's forces.

"This is just the beginning of a series of defections on thediplomatic level. We are in touch with several ambassadors,"said Mohamed Sermini, a member of the main opposition umbrellagroup, the Syrian National Council.

The defection of Fares, a Sunni, could be a major blow toAssad, who wants to convince a sceptical world he is conductinga legitimate defence of his country against foreign-backed armedgroups bent on toppling the government.

Fares' decision to jump ship follows the high-profile flightfrom Syria last week of Brigadier General Manaf Tlas, also aSunni and once a close friend of Assad, whose minority Alawitesect has relied on Sunni allies to maintain control of Syria'smajority Sunni population.

Tlas fled to Paris and has not spoken since of hisintentions. Opposition sources said Fares was leaving Iraq butit was not clear where he would go.

"The apparent decision of yet another senior Syrian officialto ditch Assad would be a chink in his armor," said one U.S.official, speaking on condition of anonymity.

"Although Ambassador Fares is not a member of Assad's innercircle, he's a respected Sunni figure and such a courageous actcould help sway other Sunni elites to follow in his footsteps."

DIVIDED ON SANCTIONS

The apparent crack in Assad's diplomatic ranks came asinternational diplomacy inched along, plagued by divisions overwhat the next steps should be to address Syria's crisis.

Annan, appointed mediator by the United Nations and the ArabLeague, briefed the Security Council by video link from Genevaon the results of this week's diplomatic shuttle to Damascus,Tehran and Baghdad - three capitals forming a Shi'ite Muslimaxis of power in the Middle East.

The deeply divided council must decide the future of a U.N.observer mission in Syria, known as UNSMIS, before July 20 whenits 90-day mandate expires. It initially approved 300 unarmedmilitary observers to monitor an April 12 ceasefire, whichfailed to take hold, as part of Annan's peace plan.

"He (Annan) called for the Security Council members to putaside their national interests and to put joint and sustainedpressure on both parties with clear consequences fornon-compliance," Britain's U.N. Ambassador Mark Lyall Grantsaid.

Russia and China, both veto-wielding permanent councilmembers, have for months blocked moves by western and Arabcountries aimed at increasing the pressure on Assad, leaving thecouncil deadlocked.

Britain on Wednesday circulated a draft resolution to extendthe monitors' mandate for 45 days and make compliance withAnnan's transition plans for the country enforceable underChapter 7 of the U.N. Charter, which allows the council toauthorize actions ranging from diplomatic and economic sanctionsto military intervention.

The draft resolution in particular threatens the Syriangovernment with sanctions if it does not stop using heavyweapons and withdraw its troops from towns and cities within 10days of the adoption of the resolution.

The British text, drafted in consultation with the UnitedStates, France and Germany, counters a draft resolutioncirculated by Russia on Tuesday, which would extend UNSMIS forthree months but makes no threat of sanctions.